People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

Optical Illusions: An Easy Way To Get Hallucinogenic Effects Without The Dangers Of Drugs

Hallucinations are all about bending one’s perception of reality. When someone hallucinates, he or she sees images, hears sounds, and feels sensations that seem real but in actuality don't exist. Optical illusions, such as the one below, are a fun, easy, and completely safe way to bring about visual hallucinogenic effects.

Optical illusions have been around for a long time. They've been traced as far back as the fifth century philosopher Epicharmus who was among the first to believe that even if our mind knows and understands everything clearly, the sensory organs deceive us. Optical illusions trick your eyes as your brain tries to interpret external information. "An illusion is a phenomenon in which our subjective perception doesn't match the physical reality of the world," Susana Martinez-Conde, director of the visual neuroscience lab at Barrow Neurological Institute, told ABC News.

Some people use drugs to achieve similar hallucinogenic effects. It is believed that these drugs help to increase creativity or have a therapeutic effect, but there is no concrete evidence as of yet to suggest this, Psychology Today reported. Still hallucinogenic drugs are seldom life-threatening and have been a part of human culture for thousands of years. Our ancestors would take hallucinogenic drugs in order to obtain mystic insight during religious rituals, as medicine, or in social gatherings.

It was not until the development of LSD, a synthetic hallucinogenic drug, in the 1960s, that abuse of these substances became a widespread and apparent problem. Today, many common hallucinogenic drugs such as LSD, ecstasy, PCP, ketamine, and DXM are created using synthetic chemicals not found in nature.